BCS theory
/ bee-see-ESS THEER-ee /
For nearly fifty years after superconductivity was discovered, no one could explain how a current could possibly flow forever. The puzzle was deep: electrons repel one another, so what could ever make them cooperate so completely? The answer, when it finally came, was both subtle and beautiful, and it tied the whole phenomenon to the gentle quivering of the atoms in the crystal.
BCS theory, named for John Bardeen, Leon Cooper, and Robert Schrieffer who published it in 1957, explains superconductivity through electron pairing. Its core idea is that vibrations of the crystal lattice provide a faint indirect attraction between electrons, binding them into Cooper pairs; all the pairs then condense into a single shared quantum state that moves as one. Because breaking any pair costs a fixed amount of energy — an energy gap — small disturbances simply cannot scatter the current, and resistance disappears.
This matters because it was the first complete microscopic explanation of superconductivity, and it correctly predicts the energy gap, the critical temperature, and many fine details for ordinary superconductors. The honest limitation is that BCS theory assumes the lattice vibrations are the glue, and that works only up to modest temperatures. The high-temperature copper-oxide superconductors discovered in 1986 do not fit the standard BCS picture, and explaining them remains one of the great open problems in physics.
BCS theory predicts that a superconductor's energy gap should be about 3.5 times its critical temperature in energy units — a number borne out beautifully by measurements on lead, tin, and aluminium.
A near-universal ratio between gap and critical temperature is one of BCS theory's clean successes.
It is easy to assume lattice vibrations always cause the pairing, but BCS is really a framework about paired condensation; in some materials a different glue, such as magnetic fluctuations, may bind the pairs, while the broad logic of pairing and a gap still applies.